The piezoelectrically active ceramic layers of the monolithic multilayer system of the known flextensional transducer are made of different lead zirconate titanate (PZT). The ceramic layers exhibit different d31 coefficients. Under the effect of an electric field, this produces in each case a dimensional change (contraction) in the relevant ceramic layer along a surface (lateral) stretch of the ceramic layer. The d31 coefficient is a measure of the inducible dimensional change and depends, for example, on the ceramic material, the layer thickness and the polarization of the ceramic layer.
In the multilayer system, there is no step change in the d31 coefficients between the ceramic layers in the stacking direction of the multilayer system. Instead the d31 coefficient varies continuously in the stacking direction, i.e. there is a d31 coefficient gradient in the stacking direction.
As a result of the different d31 coefficients of the ceramic layers, or d31 coefficient gradient, deformation (deflection) of the multilayer system occurs when the ceramic layers are exposed to approximately identical electric fields. A stroke is produced in the stacking direction of the multilayer system.
The actual dimensional change in each ceramic layer and therefore any measure of the deflection or stroke of the multilayer system depends on the strength of the electric field induced in the ceramic layer. To generate the electric fields, the monolithic multilayer system has two electrode layers between which the two piezoelectrically active ceramic layers are disposed. The electric fields in the ceramic layers cannot be generated independently of one another. It is therefore relatively difficult to produce a specific deflection of the multilayer system.
Flextensional transducers of the “Bimorph”, “Cymbal”, “Moonie”, “Multilayer” and “Rainbow” type are known from A. Dogan et al., IEEE Trans. Ultrason., Ferroelectr., Freq. Contr., Vol 44, No. 3, 597-605, 1997. To increase an effective stroke, a plurality of flextensional transducers of one of the types can be arranged to form a stack, each individual flextensional transducer of the stack contributing with its deflection or stroke to the effective stroke of the stack. The flextensional transducers of the types mentioned above have a multilayer system consisting of at least one piezoelectrically active and at least one piezoelectrically inactive layer. To produce the deflection, a marked voltage swing is generated at an interface between the layers. However, the marked voltage swing reduces the reliability of the flextensional transducer. If the flextensional transducer is continuously stressed, this can easily result in failure of the flextensional transducer.